Once upon a time, he was known as Capt. Bob, the man who trained the dolphins and seals to perform for crowds in Atlantic City. But as he got to know the dolphins, Bob Schoelkopf renounced keeping them in captivity and devoted the rest of his life to rescuing stranded marine mammals. Now the man who once prodded dolphins to jump and wave their fins for crowds is part of a broad scientific effort to determine why dolphins are dying by the hundreds.